1.Field of the Invention
The present invention relates to a full-color organic EL display, and more particularly, to an organic EL display improved in a light extraction portion.
2.Description of the Related Art
Organic EL displays generally comprise a glass substrate, a transparent electrode (high-refractive-index section) as an anode, a hole injection layer (hole transport layer), an emission layer formed of an organic film, and a cathode. The transparent electrode has a refractive index of about 2,which is greater than the refractive index, 1.5 to 1.7,of the glass substrate or the organic film (emission layer). Accordingly, the transparent electrode inevitably serves as a waveguide for guiding light therein, which causes about 50% light loss in the electrode. As a result, the light extraction efficiency of the organic EL displays is as low as about 18% at present.
To extract, to the outside, the light propagating through the transparent electrode, there is a technique for providing, on a transparent electrode of indium tin oxide (ITO), a diffraction grating made of a material having substantially the same refractive index as ITO, and extracting the light propagating through the electrode, using the grating. This technique is disclosed in Applied Physics Letters, 3779, vol. 82, 2003.
However, the diffraction grating diffracts light of particular wavelengths determined from the period size of the grating, namely, does not diffract all visible light (with wavelengths of 400 to 700 nm). Therefore, the method for providing a single diffraction grating on the front surface of a display is not necessarily effective.
To overcome this problem, there is a method for providing plurality of diffraction gratings respectively corresponding to the wavelengths of R, G and B pixels (see Jpn. Pat. Appln. KOKAI Publication No. 2003-163075). In this method, however, several types of diffraction gratings respectively corresponding to pixels must be prepared, which makes the manufacturing process very complex and hence the resultant products very expensive. It is difficult to put the method into practical use.
Furthermore, in general organic EL displays of a low-molecular-weight type, R pixels have a lower brightness than G and B pixels. To balance the brightness of the R, G and B pixels, much power must be supplied to the R pixels. This increases the power consumption of the displays, and reduces their lifetime.
As described above, in full-color organic EL displays, to enhance the light extraction efficiency, it is necessary to prepare diffraction gratings of sizes (pitches) corresponding to R, G and B pixels. This inevitably makes the manufacturing process very complex and the resultant products very expensive.